This SBIR Fast-Track application focuses on the development of a nanoconstruct for co-delivery of siRNA and chemotherapy drugs to drug-resistant human epidermal growth factor receptor type 2-positive (HER2+) breast cancer. HER2+ breast cancer is a subtype that presents HER2 overexpression and accounts for 15-25% of invasive breast cancers. HER2+ breast cancer has poor prognosis despite the rigorous current first-line treatment using two HER2-targeted therapies (trastuzumab and pertuzumab) and one taxane (paclitaxel or docetaxel). The drug combination requires 6 hours of infusion time, costs over $250,000 per treatment course, and has many severe adverse events, while achieving a progression-free survival of only 18 months. To address these shortcomings, we seek to develop a single therapy that can replace the three-drug combination. We will incorporate a taxane onto our recently developed nanoconstruct for targeted delivery of siRNA against HER2 (siHER2). The nanoconstruct is a hybrid of a 50-nm mesoporous silica nanoparticle (MSNP) and a co-polymer coating. It is conjugated with HER2 antibodies for targeted delivery. SiRNA is loaded last and was fully protected under PEG layer from blood enzymatic degradation for 24 hours (vs. <30 min as free siRNA). The siHER2-nanoconstruct reduced >80% HER2 protein levels, and was shown to overcome cancer resistance to trastuzumab and pertuzumab, which only block HER2 activity. It induced apoptotic death of HER2+ cells, but was safe to HER2- or organ cells in culture. The nanoconstruct had an excellent safety profile in blood, immune cells, general health, kidney and liver of mice. Sol-gel MSNP synthesis and layer-by-layer surface modification afford high synthesis reproducibility and scalability. MSNP is also known for its high surface area and silanol chemistry, suitable for loading hydrophobic drugs like taxanes. In mice, the siHER2-nanoconstruct inhibited growth of orthotopic tumors resistant to trastuzumab and paclitaxel but did not eradicate the tumors. Thus, as in the first line therapy, we incorporated a low dose of taxane onto MSNP core of the siHER2-nanoconstructs, which maximized cancer cell death in vitro. In Phase I (Aim 1), we will further optimize our nanoconstruct in terms of taxane and siHER2 loading and screen them for optimal size, drug release profile, efficacy in HER2+ cancer cells, and safety to normal cells. In Phase II, the optimized nanoconstruct will be evaluated for in vivo efficacy and PK profile in Aim 2 and the safety to blood, immune cells, organ function and overall health of mice in Aim 3. Both primary orthotopic tumor and metastasis models will be utilized, with the aforementioned first-line therapy as the benchmark. Results will provide important data towards an investigational new drug (IND) application to the FDA. This project is a collaboration between PDX Pharmaceuticals, LLC, specialized in material optimization, characterization, and screening in vitro, and the Dept. of Biomedical Engineering and Knight Cancer Institute of the OHSU School of Medicine, specialized in cancer biology and drug evaluation in tumor models.